In most countries, the positional accuracy of National Forest Inventory (NFI) plots is unknown, even though the NFI is expected to provide geo-referenced ground data for remote sensing. The present study evaluated the positional error of Japanese NFI plots that had already been located using a Global Positioning System (GPS) for navigational assistance toward target coordinates on a 4-km grid. The actual positions of 82 plots in 4 prefectures were measured using a high-precision GPS with dual-frequency carrier-phase observation in comparison with the target coordinates. The positional error was found to be significantly related to GPS types that had been used for navigation during the establishment of the plot, but not with terrain, canopy types and stand density. The mean error when using differential GPS (DGPS) navigation was 6.2m; significantly lower than the 20.0m obtained using autonomous GPS. We conclude that the positional accuracy of the NFI plots in Japan is acceptable for use as a ground reference for the widely used Landsat satellite images with 30-m resolution if the plots had been located using DGPS navigation. In addition, the position of plots located without the use of DGPS should be re-measured to enhance the availability of NFI data.